King Geunchogo is kinda of like the prologue to King Gwanggaeto the Great. King Geunchogo show's the events that happen before king Gwanggaeto was born
Both are stories about great kings of Goguryeo/Balhae, rich in action, struggle, strategies and overcomeing often impossible obstacles. Both main characters are strong and righteous without becoming boring.
Both are the same story of the same person with legend narrating the story with fantasy elements. The story focus on Damdeok later, Gwanggaeto the Great. Although the story is entirely different legend is watchable once.
Gwanggaeto name is referenced a few times in emperor Wang gun, if you are interested of what they are referring to, go watch the show. both shows produced by the same TV network, you will see some of supporting casts are in both shows.
Very similar history another one similar in tenchijin
Both three are about invasions and war.
About the King's wanting to protect their people
Tenchijin is Japanese with similar history but from the Japanese point of view
Gwanggaeto the Great is almost the same integrity
Saga about great emperor of Korea.
Gwanggaeto is descendant of Jumong.
The vibe from this two drama are very similar and you can connect when Gwanggaeto talks about history of his Kingdom bcs some of them can be found in drama Jumong.